The present invention relates to a digital sun sensor for measuring the attitude of an artificial satellite of three-axis stabilized type of spin-stabilized type.
The digital sun sensors previously used can be classified into the following two types:
1. Sensors having a V-slit PA1 2. Sensors having Gray-code patterns.
The sun sensor of the first type is designed for use in spin-stabilizd satellites. As is shown in FIG. 1, it has two elongated sensors 11 and 12 each comprising a slit and a photocell (not shown). The first sensor 11 extends in parallel to the spin axis S of the satellite. Second sensor 12 is inclined at a predetermined angle to spin axis S. Sensor 12 is so positioned that, in the plane containing the center of the satellite and being perpendicular to spin axis S, angle .theta. is defined by the line joining the enter of satellite and the intersection of the plane and the slit of first sensor 11 and the line connecting the center of the satellite and the intersection of the slit of second sensor 12 and this plane. First sensor 11 outputs a pulse signal shown in FIG. 2A as sunbeams pass through its slit while the satellite is spinning. The second sensor 12 generates a pulse signal shown in FIG. 2B as sunbeams pass through its slit while the satellite is spinning. The cycle Ts of the either pulse signal and the phase difference Tss between the pulse signals are measured. The angle at which spin axis S is inclined to the sunbeam SB (hereinafter called "sunbeam angle"), that is, the attitude of the satellite with respect to the sun, can be calculated from cycle Ts and phase difference Tss.
It is difficult to set angle .theta. correctly and to incline second sensor 12 exactly at the predetermined angle to spin axis S. Therefore, it is impossible for the sun sensor of the first type to detect the sunbeam angle to the accuracy of one degree or less. To make matters worse, the sun sensor of the first type cannot be used to detect the attitude of three-axis stabilized satellites.
The sun sensor of type 2 is designed for use not only in three-axis stabilized satellites, but also in spin-stabilized satellite. As is illustrated in FIG. 3, this sun sensor comprises rectangular prism 31 made of quartz or optical glass. Opaque films 33 and 35 are formed on the broadest opposing surfaces, respectively. Opaque film 33 has slit 32 cut in it by etching and which extends parallel to the axis of prism 31. A plurality of Gray-code patterns 34 are formed in opaque film 35 by means of etching. Each Gray-code pattern consists of a plurality of segments. The sun sensor further comprises arrays of photocells 361, provided in the same number of Gray-code patterns 34 and located below these patterns 34, respectively. The sun sensor is installed on a three-axis stabilized satellite such that slit 32 extend perpendicular to spin axis S of the satellite. Sunbeam SB enters prism 31 though slit 32, forming an elongated light spot on Gray-code patterns 34. The output signals of photocells 361 are input to signal-processing circuit 37.
More specifically, sunbeam SB incident on opaque film 33 at angle .theta. is refracted by prism 33, and then passes through one of the segments of each Gray-code patterns 34. Hence, sunbeam SB reaches photocells 361 located below these segments of Gray-code patterns. These photocells thus outputs "1" signals, whereas all other photocells 361 output "0" signals. Signal-processing circuit 37 identifies the segments receiving the refracted sunbeam SB, thereby obtaining angle SD corresponding to angle .theta..
Besides Gray-code patterns 34, sign-bit pattern 38 and sun pulse-generating pattern 39 are formed on opaque film 35. Pattern 39 is used when the sun sensor is used in a spin-stabilized satellite; it determines whether or not sunbeam SB has been applied to the sunbeam-receiving surface of the sun sensor. Two photocell-arrays 362 and 363 are provided below sign-bit pattern 38 and sun pulse-generating pattern 39. Upon receipt of the light beams passing through patterns 38 and 39, one of the photocells of array 362, and one of the photocells of array 363 generate "1" signals. Circuit 37 processes the output signals of the photocells, thereby producing sun pulse signal SP and the like.
The digital sun sensor of the second type is easier to set on a satellite than the sensor of the first type . Sunbeam SB falling on the earth is a light beam diverging at 0.5.degree.. Hence, even if the segments of each Gray-code patterns are arranged at intervals corresponding to 0.5.degree. or shorter intervals, angle .theta. (or angle SD) cannot be measured in the unit of less than 0.5.degree.. In order to raise the accuracy of the measuring, an additional Gray-code pattern may be interposed among the Gray-code patterns, such that the leading edges of the segments in each additional pattern are aligned with the midpoints of the segments of the adjacent Gray-code pattern. If this method is used, the layout of the Gray-code patterns will become very complex. Since it is difficult to form such a complex pattern, it is in effect impossible to measure angle SD in the unit of less than 0.5.degree..